The OncotypeDX 21-gene recurrence score assay (ODX) is the most widely used gene signature for guiding the management of patients with early stage, estrogen receptor positive (ER+), lymph node negative breast cancers. The use of ODX within the context of clinical trials and small patient populations indicates that in controlled settings, ODX likely changes management decisions in roughly 25-33% of patients and may reduce the use of chemotherapy and associated costs,toxicities, and impediments to quality of life, with one private insurer estimating savings of $1,200 per patient. Updated 2007 ASCO guidelines recommend that ODX testing may be used in patients to determine both patient prognosis and the relative benefit of chemotherapy. There is a critical need to understand which patients in the general U.S. cancer population actually receive ODX testing and how it affects their treatment, outcomes, and costs, as no epidemiologic studies of its use have been conducted outside of controlled clinical trial or modeling settings. In this proposal, we investigate the use of OncotypeDX (ODX) genomic testing in a nationally representative elderly breast cancer patient population. Aim 1: Characterize Nationwide Utilization of ODX in early stage breast cancer Aim 2: Investigate differential chemotherapy utilization associated with ODX utilization Aim 3: Investigate differential health care costs associated with ODX utilization The adoption of molecular medicine into general practice relies on many factors that are not addressed within the setting of clinical trials that include access to emerging technologies, variation in guideline adherence and quality of care, costs, patient demographics, regional variation, and other unobserved patient and physician factors that may significantly impact care. The proposed research addresses the significant need to understand how molecular technologies such as the Oncotype DX genomic test are being used in actual practice and details an innovative conceptual approach using novel claims methods and physician-patient sharing network modeling in combination with conventional rigorous methodologies to examine clinically important questions on a national scale in order to optimize current clinical practice fr breast cancer patients.